The processing of a substance, such as destructive distillation, can be carried out in one of several different ways. One of them is in a continuous process such as by means of a shaft kiln, through which a solid bed of the substance being processed is passed through a zone of favourable process conditions. Another continuous process is a rotary kiln in which a mechanically mixed bed is passed through such a zone. Some other processes are essentially batch-wise, and are carried out by means of, e.g., a batch-operated kiln. If a kiln is used, the substance being processed is brought into direct contact with oxygen that is supplied with combustion air or, alternatively, the substance being processed is brought into direct contact with oxygen that is mixed with gaseous combustion products. The combustion reaction is the source of heat that is needed to create favourable process conditions. Unfortunately part of the substance to be processed is combusted here, so that the process yield is being reduced. A suitable example of a state of the art batch-operated kiln is disclosed in US2004178052A1 (“Process for flash carbonization of biomass”), incorporated herein by reference. An alternative method for processing the substance is by means of a retort. Also a retort is essentially operated batch-wise. Thereby, the substance being processed is kept separated from oxygen in combustion air, and from oxygen mixed with gaseous combustion products. Combustion reactions of the substance being processed are thus minimized. For this reason, retorts are typically suitable to provide higher product yields than kilns. A typical industrial retort, according to the state of the art, is described here below. A typical industrial retort comprises a wall that serves to contain a substance that is being processed, whereby the substance comprises a chemical compound that is being processed or a multitude of different chemical compounds that are being processed. An industrial retort has at least one opening to enable loading the substance into the retort. The opening may or may not be provided with a sealable closure. The wall and, if present, the sealable closure ensure that the substance contained by the retort is prevented from being exposed to foreign matter. Exposure to the foreign matter during the processing of the substance might cause the substance to perform undesired chemical reactions such as oxidizing at least partly, thereby reducing the yield of the process, or the exposure to the foreign matter might result in contamination of the end-product by undesired compounds. The at least one opening of the industrial retort may also serve to unload the retort from reaction products in the form of solid matter or fluid matter or a combination thereof The wall of the retort may also serve to transfer heat from outside the retort to the substance that is being processed inside the retort. This type of heat transfer, according to the state of the art, is referred to as ‘indirect’. An industrial retort may also be provided by an entrance opening for the admission of a heat carrier fluidum at an elevated temperature and an exit opening for the expelling of the same fluidum, cooled down to a lower temperature. By flowing through the retort, the fluidum transfers heat to the substance that is being processed inside the retort. According to the state of the art, this type of heat transfer is referred to as ‘direct’. Direct heat transfer is usually faster than indirect heat transfer. On the one hand, a fast heat transfer enables a short batch processing cycle, and is therefore favourable to the productivity of a retort as more batches can be processed per unit of time than with indirect heating. On the other hand, however, a fast heat transfer may also be disadvantageous, e.g., with regard to the quality of the processing and thus may be a cause of inefficiency. One quality disadvantage of a fast heat transfer, may be, e.g., the occurrence of breakage of the solid product into pieces that are too small for use or for further processing in some specific applications of the solid product, as a result of thermal shock or rapid steam expansion. An example of undesired breakage of a solid product is the shattering of charcoal lumps into pieces that may fall through the holes of a furnace grate prior to proper combustion. A further disadvantage of direct heat transfer may be that the vaporized products get contaminated with heat carrier fluidum such that liquid recovery following collection of the vapours from the retort is less efficient. A retorting process that relies entirely on direct heat transfer is described by U.S. Pat. No. 2,160,341 (“Process for the carbonization of organic cellulosic materials of vegetable origin”), incorporated herein by reference. In contrast, a retorting process that relies entirely on indirect heat transfer is described by FR2765585(A1) (“Charcoal production by carbonisation of wood”). However, in view of the specific process occurring inside a retort, it is interesting to examine a balance between direct heat transfer and indirect heat transfer. This is explained here below.
As an example, consider a batch-wise operated retort, filled to maximum capacity with a batch of a substance to be processed. In this example the retort has a volume of, e.g., 5 m3, the substance is, e.g., wood, comprising single individual lumps not bigger than, e.g., 100 mm in any dimension, and the voidance of the retort volume is, e.g., 40% (in this example meaning that 40% of the retort's volume comprises gaseous spaces between the woody matter). In the example the substance undergoes a carbonisation process. The carbonisation process can be analysed in several ways that are distinct in scale with regard to place and time. U.S. Pat. No. 2,160,341, referred to above, distinguishes horizontal layers of a substance that are processed one after another from top to bottom of the retort. Subsequently, the respective layers undergo a process of heating and drying followed by the actual carbonisation. For the sake of completeness, we refer to US2004178052A1, mentioned above, which does not describe a retorting process but rather a kilning process, that also describes the process as proceeding in layers, this time from bottom to top.
None of the prior art documents examines a combination of direct and indirect heating.